1. Field of the Invention
The invention relates to a photovoltaic cell, comprising a plate shaped substrate of semiconductor material with a solar face and a connection face, wherein a first volume of the substrate adjacent to the solar face has been doped with a first polarity, substantially the remaining second volume of the substrate has been doped with a second polarity opposite the first polarity, and in which the first and the second volumes are separated by a pn-junction. Such photovoltaic cells are generally known.
2. Discussion of the Prior Art
Further photovoltaic cells of the above known kind are known comprising a number of through apertures in the substrate extending between both faces of the substrate and in which an electrically conducting plug has been positioned, second connection contacts located at the connection face of the substrate electrically connected with the second volume of the substrate, contact tracks located at the solar face of the substrate electrically connected with the first volume of the substrate and with the electrically conducting plug in the through apertures. The connecting face can also can be designated as the ‘shadow face’.
These photovoltaic cells are known as ‘metal wrap though cells’. They have the advantage that the connections only have to be made at a single face simplifying the assembly of these cells to form a solar panel. Such cells are described in the European Patent applications EP-A-2 068 369 and EP-A-1 985 233.
Photovoltaic cells, in particular those of the kind referred to above have a so called shunt resistance representing a parasitic electric conduction path between both connections. This shunt resistance is dominantly present in and surrounding the electrical connections between the solar face and the connection face of the substrate. Indeed the electrical potential generated on the solar face of the substrate is carried through the electrical conducting plugs in the through apertures to the connecting face of the substrate where this potential is available for connection. The protection against this shunt resistance is, in the prior art, realized by forming a low ohmic homogeneous and continuous doping of the first polarity in and on both faces surrounding the through aperture, as described in EP-A-1 985 233. EP-A-2 068 369 discloses a thicker passivation layer, typically thicker than 200 nm, on the connection face and the walls of the apertures.
As the process of homogeneous and continuous doping of the first polarity is optimized for the properties on the solar face of the substrate, that is among others a good electrical connection to the solar face metal grid, doping level will be quite high and corresponding sheet resistance quite low. This lower ohmic doping of the first polarity causes only a limited lateral resistance on the shadow face of the substrate and hence a limited blockage to a lateral leakage current. This leads to a shunt resistance with a value substantially lowering the efficiency of the photovoltaic cell.
The passivation layer on the walls of the through aperture is often applied together with such a passivation layer by a common process. The consequence thereof is that the properties of this passivation layer are the same on the faces of the substrate as on the walls of the through apertures. As the process for obtaining the passivation layer is optimized for the properties on the faces of the substrate, that is the avoidance of the recombination of charge carriers, the properties of the passivation layer for obtaining a good electrical insulation are suboptimal. This leads to a shunt resistance with a value substantially lowering the efficiency of the photovoltaic cell.
In particular in situations wherein the cell is located in less sunny conditions wherein the leak or shunt current is large in relation to the relatively low current generated by the decreased amount of solar light the influence of the leak current on the efficiency is significant. This is less the case in sunny circumstances wherein the generated current is larger so that the leak current has a relatively smaller influence.
To avoid these disadvantages U.S. Pat. No. 3,903,427 provides a photovoltaic cell, comprising a plate shaped substrate of a semiconductor material with a solar face and a connection face, wherein a first volume of the substrate adjacent to the solar face is doped with a first polarity, substantially the remaining second volume of the substrate is doped with a second polarity opposite the first polarity and in which the first and the second volumes are separated by a pn-junction extending substantially parallel to the main plane of the substrate, a number of through apertures in the substrate extending between both faces of the substrate and in each of which a single piece plug has been positioned of which at least a part is electrically conducting, which part is adapted to provide an electrical conducting connection between the solar face and the connection face, contact tracks located at the solar face of the substrate and being electrically connected with the first volume of the substrate and with the electrically conducting part of the plug, first contacts located at the connection face of the substrate and being electrically connected with the electrically conducting part of the plug and second contacts located at the connection face of the substrate and being electrically connected with the second volume of the substrate.
This insulating jacket substantially increases the value of the inherent shunt resistance, while further the increased shunt resistance offers the possibility to decrease the thickness of the doping layer on the shadow face of the substrate without adverse effects on the efficiency of the solar cell or even completely remove said doping layer. It is noted that the shunt resistance is primarily present in the part of the cell adjacent to the contact face. However the preparation of such an insulating jacket requires a separate processing step, increasing process time, required equipment and costs.
The aim of the invention is to provide such a solar cell which can be produced for lower costs.